This invention relates to a method of treating polymeric substrates to render them receptive to electroless plating, and more particularly to a method of accelerating polymeric substrates using an aqueous soluble oxidizing agent. Reference may also be made to copending application Ser. No. 335,935, now abandoned filed Dec. 30, 1981, and entitled "Method of Accelerating Polymeric Substrates Using Oxidizing Metal Ions and Alkyl Amines". That application is owned by the same assignee as the present invention and generally relates to the use of an accelerating solution which includes an oxidizing metal ion, a substituted alkyl amine, and a material selected from the group consisting of a mineral acid, an aqueous soluble salt or said substituted alkyl amine, and an aqueous soluble salt of a mineral acid, as well as mixtures thereof. The disclosure of that application is hereby incorporated herein by reference.
A variety of methods have heretofore been used or proposed for use in applying metallic platings to all or portions of the surfaces of polymeric plastic parts. Such processes conventionally comprise a plurality of sequential pre-treatment steps to render the plastic substrate receptive to the application of electroless plating whereafter the plated part can be processed through conventional electroplating operations to apply one or a plurality of supplemental metallic platings over all or selected portions of the plastic substrate. Conventionally, the pre-treatment steps employed include a cleaning or series of cleaning steps, if necessary, to remove surface films or contaminating substances, followed thereafter by an aqueous acidic etching step employing a hexavalent chromium solution to achieve a desired surface roughess or texture enhancing a mechanical interlock between the substrate and the metallic plating to be applied thereover. The etched substrate is subjected to one or a plurality of rinse treatments to extract and remove any residual hexavalent chromium ions on the surfaces of the substrate which may also include a neutralization step including reducing agents to substantially convert any residual hexavalent chromium ions to the trivalent state. The rinsed etched substrate is thereafter typically subjected to an activation treatment, such as in an aqueous acidic solution containing a tin-palladium complex, for example, to form active sites on the surface of the substrate, followed by one or more rinsing steps after which the activated surface is typically subjected to an accelerating treatment in an aqueous solution to extract any residual activator constituents or compounds on the surface of the substrate. The accelerated plastic part is again water rinsed and thereafter is subjected to an electroless plating operation of any of the types known in the art to apply a metallic plate such as copper, nickel, or cobalt over all or certain selected areas thereof whereafter the part is rinsed and thereafter is subjected to conventional electroplating operations.
Typical of such plastic plating processes are those described in U.S. Pat. Nos. 3,011,920; 3,532,518; 3,622,370; 3,961,109; 3,962,497; and 4,204,013; to which reference is made for further details of the processes, and the disclosures of which are hereby incorporated by reference. The present invention is also applicable to processes of the foregoing type and is specifically directed to an improved aqueous accelerating solution which provides benefits and advantages heretofore unattainable in accordance with prior art practices.
A continuing problem associated with the electroplating of polymeric substrates has been in the careful control of the activation and accelerating steps to achieve a plastic substrate which is receptive to the subsequent electroless plating solution to provide 100% coverage of a conductive metal layer which is adherent to the substrate and which is devoid of any lack of continuity of coverage or "skipping". The presence of such discontinuities or skips results in plastic parts which upon subsequent electroplating contain non-plated areas or non-uniformity in the metallic plating deposit rendering them unsuitable for the intended use.
In the commercial decorative plating of ABS or other plastic parts, there are several reasons why it is often desirable that some areas of the plastic surface remain unplated: (1) to enhance the decorative appearance of the part as in the case of decorative painting; (2) for economic reasons as in not wasting valuable metal and plating solutions on areas of the surface that will be invisible when the part is mounted or otherwise incorporated into a final assembly for use; (3) to maintain electrically non-conducting areas for insulation; and (4) to maintain non-plated areas for final fastening assembly such as ultrasonic welding of ABS. Prevention of plating is accomplished by selectively coating certain areas of the part with an electrically nonconductive stop-off paint prior to processing the part through electroless metallizing and conventional electroplating. Any of the commercially available stop-off compositions such as Perry & Derrick Lectro Bloc 73310 Black Stop-Off Paint, can be employed for this purpose. The formulation of these stop-off paints or coatings is intended to be such that they will not be activated and electrolessly metallized in subsequent processing steps. In practice, however, it is often the case that, due to improper use or application, or even due to the chemical composition of the stop-off coating, certain portions of such painted areas will absorb sufficient activating species to coatings has been found to be a problem in view of the varying degrees to which palladium from the tin-palladium activator is absorbed on these various coatings. For example, some coatings absorb only a small amount of palladium which will subsequently be removed during acceleration. However, other coatings absorb larger amounts of palladium which are not removed during subsequent acceleration. The obvious result in the latter case is the undesirable plating of areas coated with stop-off paint.
It should be noted that polymeric substrates which have been activated such as with a tin-palladium complex, for example, are not thereafter ready for electroless plating. The film formed on the polymeric substrate during immersion in the activator solutions will not effectively function as a catalyst and induce electroless plating until it is accelerated, which in the case of a tin-palladium activator, for example, is not until the stannous chloride also present in the substrate is removed. This is done by "accelerating"the substrate surface. Acceleration is accomplished by contacting the activated polymeric substrate with materials such as mineral acids which catalyze hydrolysis. In the case of a tin-palladium activator, for example, it is the hydrolysis of stannous chloride that is catalyzed. Although such treatment may also remove a small amount of the activator constituents such as palladium, a sufficient amount of accelerated residual material remains on the substrate surface for the subsequent electroless plating steps.
The possibilities for controlling the activity or strength of most accelerators and thereby overcoming undesirable plating, as briefly referred to above, include increasing the amount of air agitation used, varying concentration, controlling operating temperatures, or varying the immersion time used. However, the degree to which these parameters can be varied in a large scale industrial production installation is limited. This consequently limits the degree of change in activity of the accelerators, and thus limits the efforts to prevent undesired plating.
The present invention overcomes these problems by allowing the activity of the accelerator to be varied significantly and therefore overcomes many of the problems and disadvantages associated with processes for the plating of plastic articles, and particularly the acceleration thereof, by providing a solution which will inhibit plating on stop-off paints and plating racks (such as those coated with polyvinyl chloride or various plastisols and organisols), and which is of versatile use on a variety of conventional platable plastic materials.